1. Field of the Invention
This invention relates to a direct-coupling control device for a torque converter for an automatic vehicular transmission, and more particularly to a direct-coupling control device for a torque converter for an automatic vehicular transmission, consisting of a fluid torque converter provided with input and output members, a mechanism capable of mechanically and directly coupling the input and output members of the torque converter, a valve means inserted between the direct-coupling mechanism and a pressure supply source to control the locking force of the direct-coupling mechanism, a first fluid pressure output means which outputs a fluid pressure in accordance with the speed of the vehicle, and a second fluid pressure output means which outputs a fluid pressure in accordance with engine output.
2. Description of the Prior Art
It is known that, when the torque-amplifying function of a torque converter in a vehicle provided with an automatic transmission containing the torque converter is no longer necessary, the torque converter is mechanically coupled to reduce fluid slip losses which are peculiar thereto, and thereby improve the fuel economy. However, when this direct-coupling is performed when the vehicle is travelling at a low speed or the engine is in a low-rev region, the following defects occur. (1) First, the vehicle speed which enables the torque converter to reach its coupling point varies according to the torque applied while the vehicle is running. Therefore, when a small torque is applied, for example, while cruising at constant speed, the torque converter should be coupled after the vehicle speed at which no torque-amplifying function is required has been reduced to a low level. However, when the throttle pedal is then pressed, the vehicle speed enters the torque-amplifying region, so that it is necessary to release this direct-coupled state immediately. In this case, there is no choice but to increase the setting of the vehicle speed at which the torque-amplifying function is omitted, to prevent the occurrence of a time lag. (2) When the torque converter is coupled at a low vehicle speed, the engine vibrates greatly, so that suppressed noise is generated in the chassis. Although various types of torque absorbers have been proposed to prevent these defects, a device meeting all requirements has not yet been developed. Therefore, in order to prevent the generation of suppressed noise, there is no choice but to increase the setting of the vehicle speed at which the torque converter should be coupled. Under these circumstances, the torque converter is directly coupled only when the rotational speed of the engine (or, to be more precise, the vehicle speed, since a vehicle can be controlled more easily in practice if the actual speed ratio is converted into vehicle speed) is above a certain level.
One effective method of eliminating these defects and reducing fuel costs by operating a direct-coupling mechanism between the input and output members of a converter before the torque-amplifying function has been interrupted is the concept of the division of power, i.e. the concept of applying power to both a direct-coupling mechanism and a converter. The Assignee has previously proposed a simply-constructed direct-coupling mechanism capable of providing this power division very easily, which consists of a tapered member and a roller, and a control method therefore. In this proposed technique, the locking force (capacity) of the direct-coupling mechanism is increased in proportion to both the vehicle speed and the degree of opening of the throttle. For example, two signal pressures indicating the vehicle speed and the degree of opening of the throttle are input to a modulator valve through a high-selectivity valve. The sealability of the high-selectivity valve is high, so that no problems occur so long as it is operated normally. However, when hydraulic pressures are used as these signal pressures, and if dust should be mixed with the oil accidentally, oil leaks occur between the two signal pressures, so that the signal pressures become inaccurate. Accordingly, when these signal pressures are used as input signals for a speed-change control operation, errors occur.
In the previously-proposed technique, the pressure-receiving area of the modulator valve, which receives a signal pressure selected by the high-selectivity valve, is constant, irrespective of the signal pressure thus selected. Therefore, when the locking force characteristic of a direct-coupling mechanism is set to an optimum level with respect to the vehicle speed, it is not necessarily optimal with respect to the degree of opening of the throttle. Consequently, in order to set the locking force characteristic of a direct-coupling mechanism to a level that is satisfactory for vehicle speed as well as the degree of opening of a throttle, it is necessary to adjust the signal pressures again.
The locking force of the direct-coupling mechanism of the previously-proposed technique when the throttle pedal is pressed is constant in the low-speed region, irrespective of variations in the vehicle speed. A power-divided torque converter reaches its coupling point at a certain vehicle speed. At vehicle speeds lower than this vehicle speed, the torque-amplifying function provided increases as the vehicle speed decreases. Thus it is preferable that the locking force of the direct-coupling mechanism when the throttle pedal is pressed increases in proportion to the vehicle speed.